[Qemu-devel] Re: [patch uq/master 1/2] virtio-pci: wake up iothread on VIRTIO_PCI_QUEUE_NOTIFY

[Avi Kivity]

On 02/22/2010 05:29 PM, Anthony Liguori wrote:
> On 02/22/2010 09:16 AM, Marcelo Tosatti wrote:
> > > > Are you concerned about spurious wakeups?
> > > Yes.  Also, qemu_notify_event() is an undirected notification (wakes
> > > up all iothreads, and all devices), whereas ->handle_output() is
> > > directed (wakes up exactly what is needed).
> > >
> > > What's the underlying problem?  A new input buffer has become
> > > available, and we need to re-poll the incoming file descriptor?  If
> > > so, that's best done from ->handle_output() (either by waking the
> > > iothread or calling read() itself and perhaps receiving -EAGAIN).
> > Yes. Sure, perhaps calling read() itself is appropriate, and i see
> > your point that>handle_output contains more context for a smarter
> > decision.
> >
> > But one can argue thats an improvement on top of a dumb wakeup.
>
> Spurious calls to qemu_notify_event() also make it difficult to tell 
> when it's actually necessary to call qemu_notify_event() vs. when it's 
> just something that doesn't hurt.

One improvement in this area would be to add a context parameter (which 
eventually resolves to the underlying thread).  Currently we'd ignore it 
since we have just one iothread, but it would serve to document what's 
being polled, and later direct the wakeup to the correct thread.

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